It is generally known to manufacture infrared radiation absorbing soda-lime-silica glass by the incorporation therein of iron. The iron is generally present in the glass as both ferrous oxide (FeO) and ferric oxide (FezO) The total amount of iron and the balance between ferrous and ferric oxides has a direct and material effect on the color and transmittance properties of the glass. As the ferrous oxide content is increased (as a result of chemically reducing the ferric oxide), the infrared absorption increases and the ultraviolet absorption decreases. The shift toward a higher concentration of FeO in relation to Fe.sub.2 O.sub.3 also causes a change in the color of the glass from a yellow or yellow-green to a darker green or blue-green, which reduces the visible transmittance of the glass. Therefore, in order to obtain greater infrared absorption in glass, without sacrificing visual transmittance, it has been deemed necessary in the prior art to produce glass with a low total iron content which is highly reduced from Fe.sub.2 O.sub.3 to FeO. A low total iron content glass is generally regarded as one having less than about 0.70% by weight total iron expressed as Fe.sub.2 O.sub.3 For example, U.S. Pat. No. 3,652,303 discloses an infrared absorbing blue colored soda-lime-silica glass composition having a visible light transmittance greater than 70% at one quarter inch thickness, wherein at least 80% of the low total iron content in the glass is maintained in the ferrous state by the inclusion of a reducing quantity of tin metal or stannous chloride in the melt.
Many iron containing glass compositions additionally contain well known adjuvants such as titanium dioxide, molybdenum dioxide, and ceric oxide, for the purpose of providing ultraviolet energy absorption. These known ultraviolet energy absorbers have particular disadvantages, especially in the manufacture of automotive glazings, in that they cause the color of the glass to shift from a desirable green or blue-green to an unacceptable yellow color. Ceric oxide may be added, however, at a low enough concentration so as not to adversely affect the desirable green or blue-green color of such a glass.
U.S. Pat. No. 1,414,715 discloses the addition of 3% to 6% by weight of ceric oxide to prepare a non-iron-containing glass composition having a flesh tint. The patent additionally teaches that ceric oxide reduces the visible light transmittance of the glass.
U.S. Pat. No. 1,637,439 discloses the use of 5% to 10% by weight of ceric oxide as an ultraviolet absorber in dark blue glass compositions. The glass, which is useful for example for observing the operation of an open-hearth furnace, is made dark blue by the addition of 0.1% to 0.5% by weight of cobalt oxide. The high concentration of ceric oxide absorbs virtually all of the ultraviolet radiation which would otherwise pass through the eye protecting glass. Clearly, such a glass composition has a low visible light transmittance, and would not be useful for automotive or architectural glazings.
U.S. Pat. No. 1,936,231 discloses a colorless glass, wherein ferric oxide is added as an ultraviolet cut-off agent in quantities so small that the resultant glass retains its high visible light transmittance. The suggested total iron content is approximately 0.35% by weight. The patent further discloses that cerium compounds may be added in small quantities, as ultraviolet radiation cut-off agents, to low total iron containing glass compositions. Thus, the resultant glass compositions retain their colorless appearance and high visible light transmittance properties.
U.S. Pat. No. 2,524,719 discloses a rose colored glass composition, wherein iron is added to the glass batch as an infrared energy absorber, and selenium is added as an ultraviolet radiation absorber. It is suggested that ceric oxide may be included, at an amount in excess of 3% by weight, to assist the selenium in the absorption of ultraviolet radiation.
U.S. Pat. No. 2,860,059 discloses an ultraviolet absorbing glass composition, having a low total iron concentration, which is described as superior in visible light transmittance to the greenish-blue glasses generally used in automotive and architectural glazings. The maximum iron content is 0.6% by weight, in order for the glass to maintain its colorless appearance and high visible light transmittance. Titanium dioxide, and up to 0.5% by weight ceric oxide, are added to the glass for the purpose of providing ultraviolet radiation absorption.
U.S. Pat. No. 2,444,976 discloses a golden colored glass particularly adapted for glazing aircraft, having an exceptionally low transmittance in the ultraviolet and a high transmittance in the visible. The glass contains iron oxide as a heat absorbing component together with large amounts of both cerium oxide (1.5% to 3%) and titanium oxide (6% to 9%).
Finally, U.S. Pat. No. 4,792,536 discloses a process for producing an infrared energy absorbing glass, containing a low total iron concentration which is highly reduced to FeO. It is further disclosed that the infrared energy absorption can be increased by including greater amounts of total iron in the glass composition, but states that the visible light transmittance would thereby be reduced below levels considered adequate for automotive glazings. The disclosed process utilizes a two stage melting and refining operation, which provides highly reducing conditions so as to increase the amount of iron in the ferrous state for a given low total iron concentration of from 0.45% to 0.65% by weight. The patent teaches that the iron must be at least 35% reduced to FeO. Most preferably, greater than 50% of the total iron content must be reduced to the ferrous state. It is further disclosed that 0.25% to 0.5% by weight of ceric oxide may be added to the highly reduced, low total iron containing glass, for the purpose of absorbing ultraviolet radiation. It is disclosed that higher concentrations of ceric oxide are to be avoided, as they would compromise the overall transmittance properties of the glass. As an example of the glass which may be produced by the process taught in U.S. Pat. No. 4,792,536, Composition No. 11 discloses a low total iron containing glass, which is 30% reduced to FeO, and contains 1% ceric oxide. At a thickness of 4 mm, the total solar energy transmittance is about 52%, and the ultraviolet radiation transmittance is about 37%. The relatively high total solar energy transmittance value results from the low total iron concentration, while the relatively high ultraviolet radiation transmittance value is caused by the low concentration of total iron expressed as Fe.sub.2 O.sub.3, a large portion of which has been reduced to FeO.
It is common in the glass industry to refer to the total iron contained in a glass composition or batch as "total iron" or "total iron expressed as Fe.sub.2 O.sub.3 ". When a glass batch is melted, however, some of this amount of total iron is reduced to FeO, while the rest remains Fe.sub.2 O.sub.3. The balance between ferrous and ferric oxides in the melt is a result of the oxidation-reduction equilibrium, and is expressed herein and in the appended claims as the "ferrous value". Reduction of Fe.sub.2 O.sub.3 produces not only FeO, but oxygen gas as well, thus decreasing the combined weight of the two iron compounds in the resultant glass product. Consequently, the combined weight of the actual FeO and Fe.sub.2 O.sub.3 contained in a resulting glass composition will be less than the batch weight of the total iron expressed as Fe.sub.2 O.sub.3. For this reason, it shall be understood that "total iron" or "total iron expressed as Fe.sub.2 O.sub.3 ", as used herein and in the appended claims, means the total weight of iron contained in the glass batch before reduction. It should further be understood that "ferrous value", as used herein and in the appended claims, is defined as the weight percent ferrous oxide in the resultant glass divided by the weight percent of total iron expressed as Fe.sub.2 O.sub.3.
Ceric oxide is a powerful oxidizer, and when added to an iron containing soda-lime-silica glass batch composition, greatly affects the balance between ferrous oxide and ferric oxide. Carbon may be added to the glass batch to compensate for the oxidizing effect of the ceric oxide. However, high amounts of carbon have a detrimental effect on the batch melting process, as carbon preferentially reacts with batch sulfates such as salt cake or gypsum which are standard additives to soda-lime-silica glasses to accelerate silica dissolution and also act as fining agents. Thus, excessive carbon is known to cause silica scum formation during the melting of the batch and silica inclusion faults in the finished glass product.
It is known that, in order to maintain the ferrous value and therefore the green color of the glass, the amount of carbon required to counteract the oxidizing effect of about one weight percent of ceric oxide in a typical low total iron containing soda-lime-silica glass produced by the float glass process is generally in the range of 0.9 pounds of carbon per 1,000 pounds of glass. This level of carbon, however, interferes with the "silica wetting" action of the salt cake or gypsum, and thereby results in silica scum formation during the melting process and silica inclusion faults in the final product, as discussed hereinabove.
In order to maintain a constant ferrous value while counteracting the oxidizing effect of a constant amount of ceric oxide as the iron content of soda-lime-silica glass is increased to that of a high iron containing glass, for example to about 0.8% total iron, it is predicted either that the same amount of carbon must be added because the ceric oxide level is constant, or that the carbon requirement will be even greater because the equilibrium ferrous value decreases with increased iron addition, as disclosed in N.E. Densem and W.E.S. Turner, "The Equilibrium Between Ferrous and Ferric Oxides in Glasses", Journal of the Society of Glass Technology, vol. XXII, no. 914, Dec. 1938, pp. 372-389. Thus, it is predicted that a batch composition for producing a green-colored glass having high infrared energy absorption due to a high FeO concentration (from the partial decomposition of Fe.sub.2 O.sub.3 in the high total iron containing batch), and high ultraviolet radiation absorption partially due to a high concentration of ceric oxide (which is not so high as to cause the glass to appear yellow) and partially due to the large amount of Fe.sub.2 O.sub.3 which remains in the higher oxidized state, will suffer from silica scum formation during melting and result in glass having silica inclusion faults, due to excessive carbon addition.
It would be desirable to produce a green-colored glass, utilizing conventional float glass technology, for use in automotive and architectural glazings, having a high Illuminant A visible light transmittance of at least 70%, a low total solar energy transmittance of less than about 46%, and a low ultraviolet radiation transmittance of less than about 36%, at a nominal glass thickness from about 3.2 to 5 mm. It should be understood that the recitation of glass thickness, as used herein and in the appended claims, means total glass thickness for a glazing unit, which may be composed of a single glass sheet or two or more glass sheets, the total thickness of which is in the indicated range. The prior art suggests that such a glass composition having a high total iron concentration and about 1% ceric oxide, can only be produced by including a large amount of carbon in the melt, resulting in silica scum formation and silica inclusion faults in the final product.
It must be noted that the prior art referred to hereinabove has been collected and examined only in light of the present invention as a guide. It is not to be inferred that such diverse art would otherwise be assembled absent the motivation provided by the present invention.